Cretaceous Oceanic Anoxic Event 2 in the Arobes section, northern Spain: nannofossil fluctuations and isotope events

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1 Cretaceous Oceanic Anoxic Event 2 in the Arobes section, northern Spain: nannofossil fluctuations and isotope events MIHAELA C. MELINTE-DOBRINESCU 1 *, ENRIQUE BERNÁRDEZ 2, KUNIO KAIHO 3 & MARCOS A. LAMOLDA 4 1 National Institute of Marine Geology and Geo-ecology, No , Street Dimitrie Onciul, RO Bucharest, Romania 2 Departamento de Geología, Universidad de Atacama, Copayapu 485, Copiapó, Chile 3 Institute of Geology and Palaeontology, Tohoku University, Aoba, Aramaki, 760 Sendai, Japan 4 Departamento de Estratigrafía y Palaeontología, Facultad de Ciencias, Universidad de Granada, Avda. de Fuentenueva s/n, Granada, Spain *Corresponding author ( melinte@geoecomar.ro) Abstract: The Cenomanian Turonian boundary interval of the Arobes section, northern Spain, represents the maximum depth of a relatively shallow succession. The investigated section extends within the Rotalipora cushmani and Whiteinella archaeocretacea planktonic foraminiferal zones, and from UC3 up to UC8 nannofloral zones, respectively. The Oceanic Anoxic Event 2 (OAE2) is about 16 m thick and includes a positive d 13 C excursion, from 3 up to 5.5. The first peak of d 13 C is situated towards the upper part of the Rotalipora cushmani planktonic foraminiferal zone, while the second peak of d 13 C is situated in the lower part of the Whiteinella archaeocretacea planktonic foraminiferal zone. The plateau, the youngest phase of OAE2, ends slightly above the first occurrence (FO) of the nannofossil Quadrum gartneri. Based on nannofloral fluctuation, an unstable environment is recognized from the last occurrence (LO) of the nannofossil Axopodorhabdus albianus up to the FO of the nannofossil Quadrum gartneri. Mesotrophic, eutrophic and oligotrophic nannofossils have successive peaks throughout the OAE2. In the lower part of OAE2, especially in the trough phase and second build-up, productivity increased. The calcareous dinoflagellate Thoracosphaera spp. peaks up to almost 30% in the lower part of the second build-up phase. The critical nannofloral turnover episode is characterized by impoverished calcareous nannofossil assemblages and temporary disappearances of high-fertility taxa, such as Biscutum constans and Zeugrhabdotus erectus. This shift in nannofloral assemblages starts in the last stages of OAE2; that is, towards the top of the second build-up phase and also covers the main part of the plateau phase of d 13 C. Since the discovery of carbon-rich black shales in deep-sea drilling sites from the Pacific, Indian and Atlantic oceans (Schlanger & Jenkyns 1976), many investigations have focused on oceanic anoxic events (OAEs). Several Mesozoic OAEs, such as the Toarcian, the early Aptian (OAE1a) and the Cenomanian Turonian boundary event (OAE2) are viewed as global events, widely distributed on land and oceans (Arthur & Premoli-Silva 1982; Jenkyns 1985; Schlanger et al. 1987; Bralower et al. 1997; Hasegawa 1997; Voigt 2000; Jarvis et al among many others). The occurrence of the above-mentioned OAEs has been related to increased volcanism associated with a large volcanic pulse from the Caribbean Large Igneous Province that produced major environmental and climatic changes, including perturbation in the carbon cycle, ocean circulation, sea level and geochemistry of surface waters (Jenkyns 1980; Arthur et al. 1988; Larson & Erba 1999; Kuypers et al. 2002; Keller et al. 2004; Pancost et al. 2004; Selby et al. 2009). A significant event in the Cretaceous record of the carbon isotope composition of carbonate is the rapid global positive excursion of approximately 2 (d 13 C enrichment), which occurred from the late Cenomanian to the earliest Turonian interval (Arthur et al. 1988). Extensive data have been published on the Oceanic Anoxic Event 2 (OAE2 or Livello Bonarelli), which is recognized as one of the more prominent oceanic anoxic events in the Earth s history. The OAE2 is also coincident with a significant biotic turnover, as documented in the diversity and composition of the marine planktonic faunas and floras, especially of calcareous nannofossils (Lamolda et al. 1994; Paul et al. From: Bojar, A.-V., Melinte-Dobrinescu, M. C.& Smit, J. (eds) Isotopic Studies in Cretaceous Research. Geological Society, London, Special Publications, 382, # The Geological Society of London Publishing disclaimer:

2 M. C. MELINTE-DOBRINESCU ET AL. 1999; Premoli-Silva et al. 1999; Erba 2004; Mutterlose et al. 2005; Linnert et al. 2010, 2011a). North Spain includes some of the best sections of the Cenomanian Turonian Boundary Event (OAE2) (Lamolda 1978; Paul et al. 1994; Lamolda & Gorostidi 1996; Lamolda et al. 1997). This paper presents detailed investigations of the Arobes section in NW Spain, in order to further our knowledge on the lithological, stable isotope and biotic records of OAE2. The work also presents fluctuations in d 13 C and d 18 O isotopes, as well as in calcareous nannofloral composition, species richness and abundance in the investigated region, across the Cenomanian Turonian boundary interval. Geological setting The Arobes section is exposed along a roadcut on national road 634, about 500 m west of Arobes village, Asturias region, NW Spain (Fig. 1). Geologically, it is located in the easternmost third of the so-called Central Asturian Depression (CAD), a tectonic geographically bounded unit that represents Fig. 1. (a) Location of the studied Arobes section in northern Spain. Legend: 1, Palaeozoic; 2, Cretaceous sediments of the Central Asturian Depression; 3, Tertiary sediments of the Central Asturian Depression; 4, Palaeozoic and Mesozoic sediments northwards of the Santofirme Onis Fault; SOF, Santofirme Onis Fault. Inset, the Arobes section. (b) Palaeogeographical position of the studied section (after Stampfli & Borel 2002 and Gebhardt et al. 2010): a, open marine setting; b, continental shelves; c land.

3 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN a small portion of the NW Iberian microplate. In the CAD, Cretaceous rocks are around 250 m thick and lie unconformably upon a Hercynian nonmetamorphic basement. They have been subdivided into 12 formations ranging in age from late Albian to Santonian (Bernárdez et al. 1993; Bernárdez 1994). These lithological units show a shallowmarine to deep-marine setting, with the maximum palaeodepth occurring in the Upper Cenomanian Lower Turonian La Cabaña and Las Tercias formations, respectively. Palaeogeographically, the section is located between the classic European basins; that is, the Anglo-Paris and Saxony and, typical, Tethyan basins of southern Spain (Stampfli & Borel 2002) (Fig. 1). The palaeogeographical position and relatively shallow facies of the area are of general interest to the investigation of the Cenomanian Turonian boundary study interval and, particularly, the OAE2. Biostratigraphical studies are scarce in the Cretaceous deposits of Asturias. Invertebrate macrofossils of the Arobes section indicated herein, including Upper Cretaceous inoceramids, ammonoids, rudists, brachiopods and echinoids, were studied by Bernárdez et al. (1993). Microfossil data of the CAD have been discussed in more general work (Ramírez del Pozo 1972), whereas preliminary research on foraminifers include the work of Cherchi & Schroeder (1982) and Méndez & Swain (1983) on ostracods. Lamolda et al. (2001) studied the dinocysts of the Arobes section, which they correlate with the Upper Cenomanian of the Ganuza section (Navarra Province, northern Spain), previously studied by Lamolda & Mao (1999). The dinocysts characterize an open continental shelf. The lower part of the section is dominated by the Cyclonephelium group. The upper part of the section is dominated by the Spiniferites group, linked to oxygen-rich waters. Overall, the dynocyst record throughout the section suggests a deepening of the basin, from older to younger units, but with a probable terrestrial input indicated by pollen and shallow dinocysts in the Turonian Las Tercías Formation. Recently, the first detailed monograph on microfossils and selachian teeth was published by Bernárdez (2002), further allowing correlation with macrofauna. Benthic foraminiferal assemblages of Arobes, containing taxa such as Rosalina, Fursenkoina, Praebulimina, Textularia, Haplophragmoides and Lenticulina (Oba et al. 2011), indicate a shallower palaeoenvironment than in other northern Spanish sections, such as Ganuza, that also span the Cenomanian Turonian boundary (CTB) interval (Lamolda et al. 1997). The above-mentioned foraminiferal assemblage, together with the high abundance of brachiopods, indicate that the Upper Cenomanian Lower Turonian sediments of the Arobes section were probably deposited within in an inner shelf palaeoenvironment. Material and methods Stable isotopes Bulk carbonate samples of the Arobes section were analysed for stable isotopes. In total, 41 samples were collected from 22.5 m, with a resolution averaging 3 samples/m within the interval containing a laminated black mudstone at the base and around 2 samples/m for the rest of the section (Fig. 2). Data concerning the fluctuation of the isotope d 13 C in the Arobes section were first published by Oba et al. (2011), who also investigated the content of d 13 C in brachiopod shells, except for an interval between beds 15 and 19, where the brachiopods are not present. The isotopes d 13 C and d 18 O were analysed using a Finnigan Mat Delta-S mass spectrometer. The stable carbon and oxygen isotope ratios are reported in the delta notation as the per mil ( ) deviation relative to the Vienna Pee Dee Belemnite (VPDB) standard. The overall analytical reproducibility was for carbon and +0.1 for oxygen. Calcareous nannofossils Qualitative and quantitative calcareous nannoplankton analyses were achieved on 41 samples, which were the same as those used for the isotope studies (Fig. 2). Smear slides were prepared directly from the untreated samples, in order to ensure the original composition. The calcareous nannofloral analyses were performed using a polarizing light Nikon microscope at 1600 magnification. At least 300 calcareous nannofloral specimens were counted in each smear slide, the investigation being completed for 250 fields of view (see details in Lamolda et al. 1994). Because Thoracosphaera taxa are usually fragmented, we counted as one specimen either a whole coccosphere or fragments that represented at least three-quarters of a coccosphere. The fragments that were smaller than one-half of a coccosphere were counted one by one, as described by Pospichal (1995). The preservation state was recorded as follows: G good (well-preserved specimens, easy taxonomic identification at species level); M moderate (most of the specimens could be easily identified; few nannofossils show overgrowth and/or dissolution but the taxonomic identification is not hindered); P poor (most specimens show dissolution and/or overgrowth, and specific identification is often difficult). Diversity (species richness) was estimated as number of the total taxa in each sample.

4 M. C. MELINTE-DOBRINESCU ET AL. Fig. 2. Lithology and biostratigraphy based on calcareous nannofossils and planktonic foraminifers, as well as isotope fluctuations ( 13 C and 18 O) of the Arobes section across the CTB interval. Legend: 1, limestone; 2, marl; 3, nodular mudstone; 4, mudstone; 5, interval with Hepteris septemsulcata. PF, planktonic foraminifer; LO, last occurrence; FO, first occurrence. Phase A and Phase B of 13 C isotope after Oba et al. (2011). The absolute abundance is considered as the average nannofossil number in one field of view (FOV), as follows: A abundant:.1 specimen/ field of view; C common: 1 specimen/2 10 FOV; F few: 1 specimen/11 20 FOV; R rare: 1 specimen/.50 FOV. The abundance was calculated as percentage of the total nannofloral assemblages. The quantitative studies focused on eight taxonomic groups, as follows: Watznaueria barnesiae, Eprolithus floralis, Zeugrhabdotus erectus, Biscutum constans, Thoracosphaera spp., Prediscosphaera spp., Eiffellithus turriseiffeli and Cyclagelosphaera margerelii. These taxa represent between 91.5 and 98.6% of the total nannofloras. The calcareous nannoplankton biostratigraphy follows Burnett s zonation (Burnett 1998). Results Lithostratigraphy and macropalaeontological content The Arobes section displays a continuous rock sequence from the middle Cenomanian to the Coniacian. The observable part of the succession

5 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN is 50 m thick (Bernárdez et al. 1993; Bernárdez 2002) and consists, from bottom to top, of: 2 m of grainstone chalk, topmost of the La Manjoya Formation (middle Cenomanian); 9 m of mostly grey marls and mudstones of the La Cabaña Formation (Upper Cenomanian); the base of the La Cabaña Formation (which was not investigated in this study) showed a distinct unconformity, which is marked by an erosion/dissolution surface on middle Cenomanian chalk covered by a very thin ferruginous crust, 1 2 mm thick, encrusted by oysters; 22 m of glauconitic, nodular mudstone alternating with marl, lower part of the Las Tercías Formation (uppermost Cenomanian basal middle Turonian); 10 m of fine-grained, glauconitic, grainstone chalk from the upper part of the Las Tercías Formation (middle Turonian), mostly inaccessible; about 7 m of white, bioclastic chalk with rudists belonging to the Infiesto Formation, Upper Turonian Coniacian, almost entirely inaccessible. In this work, we present the results of a 22.5 m-thick part of the studied section, comprising the La Cabaña Formation and the lower half of the Las Tercías Formation (Fig. 2). The oldest studied 5.25 m (¼in the La Cabaña Formation), between sample ARO1 and ARO14A (Fig. 2), consists of several bedded couplets with dark grey marl below and paler grey mudstone above (Fig. 2). No ripples or other current structures occur in this section, although they are common in equivalent sections a few kilometres westwards, closer to the palaeocoastline. The macrofaunal content is poorly diversified, with abundant Rhynchostreon suborbiculatum, scarce neitheids and some internal moulds of gastropods. Several ammonoid taxa, belonging to the species Neolobites vibrayeanus and Metoicoceras geslinianum (R. Santamaria pers. comm to E. Bernárdez), occur only in the lowermost couplet (sample ARO1). Notably, the LO (last occurrence) of the Tethyan Late Cenomanian ammonite Neolobites vibrayeanus (El-Hedeny 2002) was recorded in a shallow-water section from Egypt towards the lower part of Rotalipora cushmani planktonic foraminiferal zone, below the first peak of d 13 C, slightly below the onset of OAE2 (Gertsch et al. 2008). Bioclasts are well preserved and diversified, although not very abundant, and they include selachian teeth, echinoderm fragments, ostracods and planktonic foraminifers. The superjacent 2 m (between samples ARO14A and ARO15C) consist of grey marls at the base, followed by nodular mudstone at the top, 70 cm thick (Fig. 2). Macrofaunal remains are very abundant and diversified in the uppermost 1.5 m, including the first occurrence of the serpulid worm Hepteris septemsulcata (Fig. 2), a typical constituent of the cold Pennrich faunas (Tröger 2003; Voigt et al. 2006), mainly composed of serpulids, brachiopods, bivalves and small oysters belonging to the genera Pycnodonte and Amphidonte. Of note is that the type locality of the Pennrich Fauna is a quarry at Dresden-Pennrich but it is widely distributed in the Elbe Valley, and is also observed in the Regensburg area, Czech Republic and Poland (Häntzschel 1933; Uhlig 1941; Tröger 2004); this type of fauna is similar to the Late Cenomanian fauna from the higher part of the Plenus Marls of southern England (Jefferies 1962). At the top of the nodular mudstone (sample 15C), a distinctive erosion surface can be traced along the entire CAD, where it is overlain by a conspicuous couplet: a lag deposit and a thin anoxic layer. The lag deposit in the Arobes section consists of a highly bioclastic limestone, 2 3 cm thick, with abundant, millimetre-sized bivalve and echinoderm debris. Reworked fossils and phosphatic clasts are common in the matrix. The identified microfauna consist of a planktonic foraminiferal assemblage belonging to the Rotalipora cushmani Zone. A layer of laminated black, plastic mudstone, 3 5 cm thick (sample ARO16/0-4), without macrofossils except very scarce reworked Hepteris tubules or benthic foraminifers, overlies the lag deposit. In contrast, a planktonic foraminifer assemblage, belonging to the Witheinella archaeocretacea Zone, is relatively abundant and well preserved. Notably, the TOC (total organic carbon), which shows very low values of around 0.2% in the interval below the thin black mudstone, reaches 0.55% in this level (Oba et al. 2011). Above the black mudstone, a 1.15 m-thick grey greenish mudstone (between samples ARO16/0-4 and ARO16/ ), with parallel lamination and no bioturbation in the basal 20 cm, occurs. Bioturbation appears upwards and becomes very intense in the top. Its gradual contact with nodular chalk marks the base of the Las Tercías Formation (Fig. 2). The TOC decreases above the black mudstone to values of around 0.2% (Oba et al. 2011). The remainder of the studied section (between samples ARO17A and ARO33) is composed of nodular mudstones, mudstones, marls and limestones. The last occurrence of Hepteris septemsulcata has been recorded 1 m above the base of the Las Tercías Formation (Bernárdez 2002). The TOC remains very low, averaging 0.2%, with a peak of 0.4% in the sample ARO25 (Oba et al. 2011). Biostratigraphy The base of Arobes section is placed in the Upper Cenomanian Rotalipora cushmani planktonic foraminiferal zone, succeeded by Whiteinella

6 M. C. MELINTE-DOBRINESCU ET AL. archaeocretacea planktonic foraminiferal zone. The LO of the planktonic foraminifer Rotalipora greenhornensis, with an estimated age of Ma (Keller et al. 2004), is situated towards the base of the investigated succession. The LO of Rotalipora cushmani, with an estimated age of Ma (Hardenbol et al. 1998), was observed 7 m stratigraphically higher (Fig. 2). The first occurrence (FO) of the calcareous nannofossil Quadrum gartneri (93.2 Ma: Erba et al. 1995) was found in the middle of the Whiteinella archaeocretacea planktonic foraminiferal zone (sample ARO27, Fig. 2). The base of the studied section is situated within the UC3 zone of Burnett (1998), as supported by the assemblages containing Corollithion kennedyi and other significant biostratigraphical species, such as Lithraphidites acutus, Ahmuellerella octoradiata, Axopodorhabdus albianus, Eiffellithus turriseiffelii, Helenea chiastia and Isocrystallithus compactus (Figs 3 & 4). The oldest nannofossil event observed in the Arobes section is the LO of Corollithion kennedyi (¼the top of the UC3 zone, coincident with the top of the UC3e subzone of Burnett 1998). The next recorded nannofloral event is the LO of Axopodorhabdus albianus (Fig. 4) that is placed, in the Arobes section, slightly above the LO Corollithion kennedyi and around 5 m below the LO of Lithraphidites acutus (Fig. 4). In Burnett s zonation (Burnett 1998), the LO of Lithraphidites acutus (i.e. the top of the UC4b nannofossil subzone) is situated below the LO of Axopodorhabdus albianus. Despite the fact that it is widely accepted that A. albianus became extinct close to (below) the CTB, this event is placed in different stratigraphical positions by various authors. Burnett (1998) identified successive LOs of A. albianus and Rhagodiscus asper, followed by the FO of Eprolithus octopetalus in the latest Cenomanian. In northern Italy, the extinction of L. acutus is followed by those of A. albianus and Rhagodiscus asper (Luciani & Cobianchi 1999); in southern France, the LO of A. albianus preceded the successive LOs of L. acutus and C. kennedyi (Fernando et al. 2010); while, in northern Spain, Lamolda et al. (1997) reported the successive LOs of C. kennedyi, A. albianus and L. acutus. Recently, Paul & Lamolda (2009) demonstrated the reliability of the LOs of Corollithion kennedyi, Axopodorhabdus albianus, Lithraphidites acutus and Helenea chiastia, and the FO of Quadrum gartneri, by calibration with cyclostratigraphy in Menoyo, northern Spain, and Dover, SE England. Slightly above the LO of the foraminifer Rotalipora cushmani, the LO of Lithraphidites acutus (¼base of the UC5 zone of Burnett 1998), followed by that of Rhagodiscus asper, were recorded (Fig. 2). The above-mentioned events took place within the Whiteinella archaeocretacea planktonic foraminiferal zone, as in other sections from Tethys and Boreal realms, spanning the CTB interval (Gorostidi & Lamolda 1991; Paul et al. 1999; Fernando et al. 2010; Linnert et al. 2010). The LO of Lithraphidites acutus is followed by successive FOs of Quadrum intermedium and Eprolithus octopetalus, the earlier event being indicative for the base of the UC5c subzone of Burnett (1998). The succession of the Late Cenomanian nannofossil events recorded in the Arobes section has common features with those identified within the Cenomanian Turonian interval of the Alava (Menoyo section) and Navarra (Ganuza section) provinces, northern Spain (Gorostidi & Lamolda 1991; Paul et al. 1994; Lamolda et al. 1997). Many authors indicate that the CTB falls within the UC5c subzone, and the FO of E. octopetalus is an earliest Turonian event (Burnett 1998; Paul et al. 1999; Luciani & Cobianchi 1999; Hardas & Mutterlose 2006; Wagreich et al. 2008; Fernando et al. 2010). In terms of calcareous nannofossils, the CTB in the Global Boundary Stratotype Section and Point (GSSP) for the base of the Turonian Stage from Pueblo, Colorado, USA falls between the LO of Axopodorhandus albianus and the LO of Helenea chiastia. The latter nannofossil event is placed in an interval that falls between the FO of the bivalve Mytiloides hattini (0.3 m below the CTB that is marked by the FO of the ammonite Watinoceras devonense) and just below the FO of the planktonic foraminiferal species Helvetoglobotruncana helvetica, 0.9 m above the CTB (fig. 9 in Kennedy et al. 2005). In the Arobes section, the H. chiastia LO is situated above the FO of Quadrum gartneri. Notably, isolated specimens of Helenea chiastia (zonal marker for UC5c) were also recorded in the UC7 zone, above the FO of Quadrum gartneri (Linnert et al. 2011b). Possibly, this occurrence is linked to reworking or to a younger extinction of Helenea chiastia. The next nannofloral event observed in Arobes is the FO of Quadrum gartneri (Figs 2 & 4). Many authors (Lamolda et al. 1994; Burnett 1998; Luciani & Cobianchi 1999; Lees 2002; Erba 2004; Melinte-Dobrinescu 2010) placed the FO of Quadrum gartneri above the CTB. At the GSSP for the base of the Turonian stage, the FO of Quadrum gartneri is also situated above the CTB (Tsikos et al. 2004; Caron et al. 2006). Furthermore, the FO of Quadrum gartneri was identified in many regions (northern Spain: Gorostidi & Lamolda 1991; Lamolda et al. 1997; Eastbourne, UK: Paul et al. 1999; Tezra, Morocco: Tantawy 2008; NW Germany: Linnert et al. 2010) above the OAE2. Tsikos et al. (2004) indicated that the FO of the nannofossil Quadrum gartneri coincides reasonably well, especially in Tethyan sections, with the stratigraphy

7 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN -ARO 19 A PP C P P P C C P SAMPLES Preservation Ahmuellerella octoradiata Amphizygus brooksii Axopodorhabdus albianus Biscutum constans Biscutum coronum Braarudosphaera bigelowii Broinsonia enormis Calculites percenis Chiastozygus litterarius Corollithion kennedyi Cretarhabdus conicus Cretarhabdus striatus Cyclagelosphaera margerelii Cyclagelosphaera rotaclypeata Cylindralithus serratus Diazomatolithus lehmanii Eiffelithus eximius Eiffellithus turriseiffelii Eprolithus apertior Eprolithus floralis Eprolithus octopetalus Flabellites oblongus Haqius circumradiatus Helenea chiastia Helicolithus trabeculatus Isocrystallithus cf. I. compactus Lithraphidites acutus Lithraphidites carniolensis Loxolithus armilla Manivitella pemmatoidea Marthasterites simplex Microrhabdulus belgicus Microrhabdulus decoratus Nannoconus spp. Octocyclus reinhardtii Prediscosphaera cretacea Prediscosphaera columnata Prediscosphaera ponticula Quadrum gartneri Quadrum intermedium Radiolithus planus Retecapsa angustiforata Retecapsa crenulata Rhagodiscus achlyostaurion Rhagodiscus angustus Rhagodiscus asper Rhagodiscus splendens Staurolithites crux Stoverius achylosus Thoracosphaera spp. Tranolithus gabalus Tranolithus orionatus Zeugrhabdotus diplogrammus Zeugrhabdotus elegans Zeugrhabotus embergeri Zeugrhabdotus erectus Watznaueria barnesiae Watznaueria biporta Watznaueria manivitae Watznaueria ovata NANNOFOSSIL ZONES STAGE ARO 33 M F R F F F F F C R R F F C C F R F F R R F C F F R F F F R P R F R R F C R F ARO 31 M F F P F F F C R F C C F R P F R P R F F P C R R R F R P R F F F C F UC8 TURONIAN ARO 29 M F R F F R R F R P F C P C R P P R C P P R F R R R F F R R R F R F C p P F ARO 28/2 P R F R F P P F C R R R P F F R F P F P P R P P R P R F C C F F UC7 ARO 27 P F F R R P R F F P C R P P P R C F P R P F F R R R F R F C P R c ARO 25 P R R F P P R C R R R P F R R F R F R R F F C U C 5 ARO 23 P P R R R R P F P C P R P F R P R F F F R R F F R C R ARO 21 P R P C P F R P C R ARO 19 B P C P R R F C C U C 6 ARO 19 base P P C R P F C C ARO 17 E P R R P C R R C F C R ARO 17 D P R F P C P C C P ARO 17 C P R P F F P C P C C ARO 17 B P R P F F C P P P P C C ARO 17 A P R P P C P P R C R C ARO 16/ P R P C P P F P F C R C F ARO 16/ P R P P C P P F F C F C ARO 16/80-90 P R R P R R C P C F C F ARO 16/70-80 P R P R C P R P P F R P F F C R R F F R C P F ARO 16/60-70 P R R P F F F F F R C R P P F P F F F R P R R F C F R C UC5 ARO 16/50-60 P F R F R F R F P F R R F C P P P F P F P C R F C P P a-b ARO 16/40-50 P R R R P R R R P C R P P R R P R P R F F F F C R R F F F C P F A N ARO 16/30-40 M F R R F F F F R F C F P R P F R R P C F C F IARO 16/23-30 MF R P F F F C P P R P F R P C P R F C P F ARO 16/15-23 M F R F P R P C R P F P P R P C F R C F F ARO 16/10-15 P F F F F P C P P P F P P P F F P F C R F R C P ARO 16/4-10 M F P R F F P C P P P P P R P R P P F F R C R R R C ARO 16/0-4 P F F R P F R P P C P P P P P P P P F P P P F P F R P C R F F R C P P R ARO 15 C M F F R P R R F F P F F C R P F P F F F P P F R R R F F P C R F R R C P ARO 15 B M F R P R R R R F F C P F R F F P P F F C R F R R C R F ARO 14/B M R F R P F F R P R F C R P F F R R F R F P F P R F P F R C P P F R R C R F ARO 14/A M F F P P F F F R C R R P R F R F F R C F P F R F R C P R F F C ARO 13 B M P F F P F R R F P R F C F R F F R F P F F C R F C UC4 ARO 12 M P F P F F R R R C R F C R C R F R F R P R F R F R R C F F R R R R F F F F A F ARO 10 M F R R F R C R R C R C P R R R F F R R P C R P F R R R F R R R F R C R R ARO 8 M F R R P R C P P F C P C R R R R R P R R R R C R R R P F R R F P R R R C R R ARO 6 G R F P F F R C R R C R R C R C F R R F R R R R F F F C F R F P R F R P F F R R R F F C R R R ARO 4 G R R F F F P F F C F R F C F A F R R F R R R P R C F R R R R F R F F R F R R F R R F C F P R ARO 2 G R F F C C F P F F C F F F C F A R F F F F F F F P C F F C F R F F F F R R R F F F F R F A P F UC3 ARO 1 G P F F F C C C R F F C F F F C R C F F F C C F F P F C R F C F F F F F F R F R R R F F C C F P F A F R C C E N O M A N Fig. 3. Calcareous nannofossil distribution in Arobes within the Late Cenomanian Early Turonian interval. Preservation: P, poor; M, moderate; G, good. Abundance: P, present; R, rare; F, few; C, common; A, abundant. UC zones and subzones after Burnett (1998).

8 M. C. MELINTE-DOBRINESCU ET AL. Fig. 4. All microphotographs LM (light microscope). Figures in crossed-nicols, except for 2, 4, 7 and 15 in transmitted light. 1, 2, Lithraphidites acutus Verbeek & Manivit in Manivit et al. 1977; sample ARO2. 3, 4, Lithraphidites carniolens Deflandre 1963; sample ARO2. 5, Axopodorhabdus albianus (Black) 1971 Wind & Wise in Wise & Wind 1977; sample ARO6. 6, Helenea chiastia Worsley 1971; sample ARO19A.. 7, Biscutum constans (Górka 1957) Black in Black & Barnes 1959; sample ARO33. 8, Manivitella pemmatoidea (Deflandre in Manivit 1965) Thierstein 1971; sample ARO21. 9, Eprolithus floralis (Stradner 1962) Stover 1966; sample ARO17A. 10, Rhagodiscus asper (Hill 1976) Doeven 1983; sample ARO15C. 11, Rhagodiscus angustus (Stradner 1963) Reinhardt 1971; sample ARO29. 12, Rhagodiscus splendens (Deflandre 1953) Verbeek 1977; sample ARO23. 13, specimens of Watznaueria barnesiae (Black in Black & Barnes 1959) Perch-Nielsen 1968; sample ARO21. 14, Quadrum intermedium Varol 1992; sample ARO16/ , Quadrum gartneri Prins & Perch-Nielsen in Manivit et al. 1977; sample ARO27. 16, Zeugrhabdotus erectus (Deflandre in Deflnadre & Fert 1954) Reinhardt 1965; sample ARO16/ , Thoracosphaera sp.; sample ARO16/ , Zeugrhabdotus embegeri (Noël 1958) Perch-Nielsen 1984; sample ARO21. 19, Eiffellithus eximius (Stover 1966) Perch-Nielsen1968; sample ARO31. based on ammonites of the CTB, as well as with the proposed end-point of the d 13 C isotopic excursion. In some sections, including the one presented herein, the FO of Quadrum gartneri took place towards the upper part of the Whiteinella archaeocretacea planktonic foraminiferal zone and at the base of the Mytiloides mytilodes inoceramid zone. While in other sections (i.e. Austria: Pavlishina & Wagreich 2012), this nannofloral event is situated slightly above the base of Helvetoglobotruncana helvetica planktonic foraminiferal zone. In general, the CTB boundary is placed in the plateau interval identified based on d 13 C isotope fluctuation, towards the upper part of Whiteinella archeaocretacea planktonic foraminiferal zone (Keller et al. 2004; Tsikos et al. 2004; Voigt et al. 2006; Pearce et al. 2009; El-Sabbagh et al among others). Taking into consideration these data, in the Arobes section the CTB could be placed in an interval between samples ARO21 and ARO25, within the UC5c UC6 calcareous nannofossil zone and subzone, below the FO of Quadrum gartneri (sample ARO27), and towards the upper part of the plateau interval yielded by the isotope d 13 C. The youngest nannofloral event of the studied section is the FO of Eiffellithus eximius, which

9 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN marks the base of the UC8 zone of Burnett (1998). This event is coincident, in the Arobes section, with the FO of Marthasterites simplex (Fig. 3). Stable isotopes Fluctuation of d 13 C. At Arobes, there is a positive excursion of the isotope d 13 C, from 3 to about 5.52, in between the base and the top of the measured section (Fig. 2). Notably, most of the studied samples from the upper part of La Cabaña Formation and lower part of Las Tercías Formation yielded values over The fluctuation pattern of the carbon isotope recognized in Arobes generally follows the phases recognized by Paul et al. (1999) based on d 13 C fluctuations in the Eastbourne section (UK). They are, from earliest to latest: (1) pre-excursion phase with low d 13 C values; (2) first build-up, characterized by a distinct rise in d 13 C; (3) trough interval with reducing d 13 C values; (4) second build-up, with a new shift of d 13 C; (5) plateau of high d 13 C values; (6) recovery, characterized by a gradual decline in d 13 C values; and (7) post-excursion phase with a gentle decline in d 13 C values. In the studied section, the increase in d 13 C indicates the onset of the OAE2 within the Rotalipora cushmani planktonic foraminiferal zone, as several authors described (Lamolda et al. 1994; Paul et al. 1994; Keller et al. 2004; Jarvis et al among others) that correlates with the top of the UC3 nannofossil zone and extends almost into the entire UC4 nannofossil zone. This interval can be assigned to the first build-up of d 13 C positive excursion described by Paul et al. (1999), expressed in Arobes by a shift from 3.2 up to 4.2 (Fig. 2). Afterwards, the trough interval with decreased d 13 C values was recorded towards the top of Rotalipora cushmani planktonic foraminiferal zone, and in correlation with the top of the UC4 nannofossil zone. This interval comprises a decrease of d 13 C from almost 5.5 (the maximum of d 13 C recorded in the CTB of Arobes) down to 4.6. The second build-up or the third phase related to OAE2 (Paul et al. 1999), showing d 13 C values of between 4.5 and 5.4, contains the top of the UC4, UC5a b, and the lower part of the UC5c UC6a b, zones and subzones, respectively. This interval contains several bioevents, such as the LO of the planktonic foraminiferal species Rotalipora cushmani, and therefore the boundary between the Rotalipora cushmani and Whiteinella archaeocretacea planktonic foraminiferal zones, as well as successive LOs of the nannofossils L. acutus and R. asper, followed by the successive FOs of the nannofossils Q. intermedium and E. octopetalus. Within this interval, most of the range of the cold-water serpulid worm Hepteris septemsulcata was observed in Arobes (Fig. 2). The next phase of d 13 C is the plateau, yielding values of between 4.8 and 5.2. The plateau extends within UC5c UC6a b and the lower part of the UC7 nannofossil zones and subzones. It develops entirely within the Whiteinella archaeocretacea planktonic foraminiferal zone. This interval, where the CTB is situated, includes no significant biostratigraphical changes, except the FO of the nannofossil Quadrum gartneri. Hence, the FO of Q. gartneri that marks the base of the UC7 biozone is placed at the end of the d 13 C plateau interval, slightly above the launch of recovery phase, and below the top of OAE2 (Fig. 2). In several regions that is, Eastbourne (UK), Pueblo, Colorado (USA), western Morocco and the Eastern Alps of Austria (Paul et al. 1999; Tsikos et al. 2004; Kennedy et al. 2005; Gertsch et al. 2010; Pavlishina & Wagreich 2012) the FO of Q. gartneri is situated towards the top of the d 13 C positive excursion marking the end of OAE2. The recovery started above the FO of the nannofossil Q. gartneri and slightly below the FO of Eiffellithus eximius (i.e. the base of the UC8 nannofossil zone). Within this interval, the d 13 C values gradually decreased, from 4.7 down to 4.2. The top of the studied section corresponds to the d 13 C post-excursion interval, with values below 4.0 and a lowermost value of 3.6 recorded in sample ARO33. Hence, the d 13 C values return towards lower pre-excursion ones, between 2.9 and 3.1 (Fig. 2). Fluctuation of d 18 O. The variation in the d 18 O isotope is between 22.3 and Less negative values recorded in the lower part of the Arobes section correspond to the pre-excursion, first build-up and trough phases of d 13 C, where there are only two d 18 O values below 25. Starting with the second built-up phase of d 13 C, a shift to more d 18 O negative values was observed. Most d 18 O values are between 24 and 25, with a minimum of 25.7 at the beginning of the d 13 C plateau phase. A cross-plot of the d 13 C and the d 18 O values (Fig. 5) shows a low correlation between the two stable isotope fluctuations (R ¼ 0.1). Diagenesis and isotope fluctuation. Several authors have indicated that primary isotopic signals can be altered by post-depositional diagenetic alteration (Banner & Hanson 1990; Jenkyns et al. 1994). In general, shallow-water carbonates are more prone to diagenesis as a result of meteoric-vadose diagenetic overprinting. Nonetheless, the d 13 C records of Cretaceous shallow-water sections have been successfully correlated with well-preserved

10 M. C. MELINTE-DOBRINESCU ET AL. Fig. 5. Carbon v. oxygen stable-isotope cross-plot in the Arobes section. Data are for bulk sediments and are given in VPDB (Vienna Pee Dee Belemnite). deep-water sections (Davey & Jenkyns 1999; Parente et al. 2007). It has been argued that oxygen isotopes are more sensitive to diagenetic effects that may lead to considerable lowering of d 18 O values (Schrag et al. 1995). In the studied section, d 18 O values show greater fluctuations than d 13 C, possibly being more affected by diagenesis, although note that several complete sections of CTB from both deep- and shallowmarine environments also yielded large fluctuations in d 18 O. For instance, the shallow-water carbonate platform of southern Mexico yielded d 18 O values of between 29.2 and 22.2, averaging 26.5 (Elrick et al. 2009). These authors suggested that the Mexican values are depleted by more than 3, relative to diagenetically unaltered marine calcite of similar age. In Pueblo, Colorado, at the GSSP of the Turonian stage, d 18 O, measured on the surface-dweller Hedbergella planispira, shows very low values, between 27 and 212, that mirrored the mixing between marine and freshwater components (Keller et al. 2004). These authors found low values towards the top of Rotalipora cushmani planktonic foraminiferal zone, a peak to positive values at the boundary between Rotalipora cushmani and Whiteinella archaeocretacea planktonic foraminiferal zones, and a negative d 18 O excursion just below the CTB, followed by more positive values within the base of the Turonian. In Crimea, across the CTB, the larger fluctuation in d 18 O and the most negative values were observed at the top of the Rotalipora cushmani and in the lower part of the Whiteinella archaeocretacea planktonic foraminiferal zones (Fisher et al. 2005). The d 18 O data show a sharp decrease in values coincident with a distinct increase in the carbon isotope values, probably linked to the increased seasurface temperatures around the time of the CTB but Fisher et al. (2005) identified that samples had undergone a degree of diagenetic alteration, based on the petrographical analysis. In general, d 18 O yielded a more striking signal than d 13 C, and this feature could also be seen in Arobes, where the most important fluctuations in d 18 O are placed in the interval between the upper part of the Rotalipora cushmani and lower part of the Whiteinella archaeocretacea planktonic foraminiferal zones, showing values of approximately 26 and 23. Although this signal may be consistent with the diagenesis, we may surmise that temperature trends may still be preserved, indicating elevated temperatures during burial diagenesis. Calcareous nannofossils Calcareous nannofossil fluctuations. A total of 60 nannofossil species have been identified in the studied samples. The greatest diversity of 52 (¼number of identified taxa) is recorded at the base of the succession, and the smallest diversity of seven taxa is observed at the end of the second build-up of d 13 C and above in the plateau phases of d 13 C, at the level of the lower part of the UC6 nannofossil zone (Figs 3 and 6). The abundance varies between 6.9 specimens/fov (at the base of the studied section) and 0.4 specimens/fov, at the boundary between the second build-up and the plateau phases of d 13 C (Fig. 6).

11 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN Eight taxonomic groups were selected for semiquantitative analysis, they include: Watznaueria barnesiae, the most abundant Cretaceous species and one of the nannofossils most resistant to dissolution. Assemblages containing more than 40% W. barnesiae are described as being heavily altered (Roth & Krumbach 1986; Lamolda et al. 1994). A high abundance of this nannofossil may also be indicative of low fertility waters (Erba et al. 1995) and high surface water temperature (Watkins 1992; Lamolda & Gorostidi 1996). W. barnesiae is viewed as an oligotrophic nannofossil, being a K-strategist taxon that was ecologically comparable to the present-day highly cosmopolitan species Emiliania huxleyi (Mutterlose & Kessels 2000; Melinte & Mutterlose 2001). During the Late Cretaceous of the Boreal Realm, W. barnesiae seems to record well any warming events (Sheldon et al. 2010). According to Linnert et al. (2011a), the occurrence of this nannofossil is more related to oceanic assemblages. Eprolithus floralis, a species that shows a distinct enrichment within the CTB (Roth & Krumbach 1986; Bralower 1988; Lamolda et al. 1994; Paul et al. 1999; Erba 2004). E. floralis is also used as an indicator of the nannofloral preservation because it is highly resistant to dissolution (Bralower 1988; Paul et al. 1994). Its blooms within the CTB interval and have been correlated with cooler climate intervals (Paul et al. 1999; Erba 2004). Conversely, its decline in abundance during the d 13 C positive excursion of OAE2 has been suggested to indicate warmer conditions during that event (Hardas & Mutterlose. 2007). Biscutum constans, a taxon that typically increases in abundance during Cretaceous OAEs (Premoli-Silva et al. 1999; Erba & Tremolada 2004), is viewed as an indicator of mesotrophic conditions (Erba 2004; Thibault & Gardin 2010) and a high fertility proxy (Thibault et al. 2012). This taxon is more common at high palaeolatitudes (Mutterlose 1992; Street & Bown 2000; Melinte & Mutterlose 2001), possibly indicating an upwelling of cold water rich in nutrients. Eleson & Bralower (2005) found a negative correlation between the percentages of B. constans and oligotrophic taxa such as W. barnesiae and Eiffellithus turriseiffelii but they documented no significant correlation with mesotrophic or eutrophic taxa such as Staurolithites spp., Zeugrhabdotus elegans and Z. erectus. B. constans abundance is favoured by nutrient-rich shelf waters affected by fluvial nutrient input or coastal upwelling (Linnert et al. 2011a). Zeugrhabdotus erectus is a proxy of highfertility surface water (Roth 1981; Mutterlose 1996) and is typical of high latitudes (Roth & Fig. 6. Nannofloral fluctuation of the selected calcareous nannofossil groups in the Arobes section. Diversity, number of total taxa; abundance, specimens/fov.

12 M. C. MELINTE-DOBRINESCU ET AL. Krumbach 1986; Herrle et al. 2003; Mutterlose et al. 2005). Blooms of this nannofossil occur during OAEs, including OAE2 (Lamolda et al. 1994; Premoli-Silva et al. 1999; Hardas & Mutterlose 2007; Linnert et al. 2010). Cyclagelosphaera margerelii is a high diagenetic and solution-resistant species (Thierstein & Roth 1991). In Upper Jurassic sediments, C. margerelii shows minor positive correlation with W. barnesiae (Pittet & Mattioli 2002) but a significant positive correlation has been demonstrated between W. barnesiae and C. margerelii in Upper Cretaceous deposits (Lamolda et al. 2005). Busson et al. (1993) suggested high trophic preferences for C. margerelii because this taxon possibly bloomed in conditions of intense nutrient and freshwater influxes in Late Jurassic deep-lagoonal environments. Prediscosphaera spp. counted in our study included P. cretacea, P. columnata and P. ponticula, which have been described as cosmopolitan (Mutterlose 1992; Lees 2002) and low-productivity indicators (Eshet & Almogi- Labin 1996). In the equatorial Atlantic, the positive correlation of Prediscosphaera spp. with Z. erectus was interpreted as a preference of these taxa for eutrophic settings (Hardas & Mutterlose 2007). Eiffellithus turriseiffelii is a nannofossil that shows a positive correlation with W. barnesiae within the CTB (Eleson & Bralower 2005). The abundance of E. turriseiffelii within that interval has a high positive correlation with Shannon diversity, thus indicating that conditions favourable for nannofloral diversity support high abundance of this species (Tantawy 2008). Taxa of the genus Eiffellithus, as well as those belonging to the Zeugrhabdotus and Prediscosphaera genera, have been considered cosmopolitan by Linnert et al. (2011a), with no particular affinity to either shelf or openocean settings. Thoracosphaera is a calcareous dinoflagellate genus that blooms under stressful marine conditions, as recorded, for instance, just above the K T (Cretaceous Tertiary) boundary (Romein 1977; Pospichal 1995; Gardin & Monechi 1998; Lamolda et al among many others) in the Lower Cretaceous sediments of the Indian Ocean (Bralower 1992) and across the Paleocene Eocene transition in the sub-equatorial Pacific (Raffi et al. 2005). Its increased abundance was interpreted to reflect unusual environmental conditions, such as large amounts of CO 2, and fluctuations in salinity and ph of surface waters, accompanied by episodes of considerable warming (Hildebrand-Habel et al. 1999). Calcareous dinocysts are solution-resistant taxa compared to other components of calcareous nannofloras (Wendler et al. 2002); hence, we may assume that the same can be applied to Cretaceous Thoracosphaera. Recent studies (Zonneveld 2004) suggest that the use of respiratory carbon for calcite precipitation could be a common feature in calcareous dinoflagellate development. Within the studied section, significant fluctuations in the eight selected taxonomic groups of nannofossils were observed. The pre-excursion interval is characterized by abundant Watznaueria barnesiae, showing almost constant percentages at around 35%, while Eprolithus floralis fluctuates very little and represents around 18% of the total nannofloras. Both taxa represent more than 50% of the total nannofloras in this pre-excursion interval (Fig. 6). Prediscosphaera spp. (mainly P. cretacea) increases slightly from 8 to 9%. Eiffellithus turriseiffelii also shows small changes in its relative abundance, which varies from 6 to 8%, but no trend could be identified. Cyclagelosphaera margerelii remains rare, at around 5 6%, showing a minor decrease in trend upwards. The high-fertility proxies, Biscutum constans and Zeugrhabdotus erectus, are both rare, averaging 2%. Taxa of the calcareous dinoflagellate Thoracosphaera are also present throughout the pre-excursion interval, representing around 5%. During the first build-up phase that coincides with the onset of OAE2, we observed a gently decline in diversity, from up to 46 to 36 taxa, and a more significant one in abundance, from 6.9 to 2.5 specimens/fov. Watznaueria barnesiae represents between 33 and 44% of total nannofloras, with a shift to 29% at the top of the first build-up phase. E. floralis shows little fluctuation, between 19.7 and 17.2, but exhibits low values (around 9%) close to the top of this phase. Biscutum constans percentages are between 2.5 and 3.8% but start to increase rapidly, up to 4.8% at the top of this interval, therefore doubling in abundance. Z. erectus is constantly low, up 2.2%, while C. margerelii decreases straight down from above 5% to less than 2%, then shows a recovery to 4.2% at the top of the first build-up phase. Prediscosphaera taxa increases from the base to the top of this interval, by 3 4%, a feature also observed in the temporal distribution of E. turriseiffelii. Thoracosphaera is present in all of the studied samples of this interval, but its abundance remains low, between 5.9 and 8.4% (Fig. 6). The trough interval of d 13 C is characterized by constant diversity and abundance values, similar to those recorded in the upper part of the first build-up phase. Within this interval, the abundance of the main components of nannofloral assemblages,

13 OCEANIC ANOXIC EVENT 2 IN NORTHERN SPAIN such as W. barnesiae and E. floralis, jointly amount to 50 60% of nannofloras (Fig. 6). The most significant fluctuation of this interval is of B. constans, which significantly increases from 2% to about 5%, while Zeugrhabdotus erectus slowly decreases from 2.2 to 1.7%. Cyclagelosphaera margerelii abundance goes up to 5%, showing values similar to those recorded within the pre-excursion and first build-up phases of d 13 C. Prediscosphaera spp. maintained the increasing trend, while the abundance of Eiffellithus turriseiffelii fluctuated between 5.5 and 7.3%. Concerning the calcareous dinoflagellate, Thoracosphaera reveals no significant change, and its percentage averages 8%. During the second build-up phase of d 13 C, and especially towards its end, there are nannofloral fluctuations in Arobes. The diversity continuously decreases from 34 taxa at the bottom to seven taxa at the top of this interval. It concurs with a strong shift in abundance from 2.7 down to 0.4 specimens/fov. The abundance of Watznaueria barnesiae increases spectacularly, from around 33% at the base of the second build-up phase of d 13 Cupto 72.2% at its top. It is associated with a large fluctuation in Eprolithus floralis, which reaches its peak, of almost 40%, at the mid level of this phase. Notably, the maximum of E. floralis is coincident with a significant decline in both absolute abundance and diversity, and with W. barnesiae minimum values in the whole studied section at around 9% (Fig. 6). Subsequently, E. floralis starts to decrease and reaches the lowest values in the studied section, of 3.2%, at the limit between the second build-up phase and the plateau phase of d 13 C. Biscutum constans continuously decreases, yielding values as low as 1.5% at the top of this phase. By contrast, Zeugrhabdotus erectus shows a peak of almost 6% at the bottom of this interval, and then decreases to values below 1%. Likewise, the trend of Cyclagelosphaera margerelii shows a substantial increase, with a peak of 7%, but almost vanished at the top of the second build-up phase. Prediscosphaera spp. and E. turriseiffelii also increased within this interval but decreased towards the end of the second build-up of d 13 C. Thoracosphaera spp. presents a variation with a double peak, up to 27.3%, in the lower part of second build-up phase, and then significantly decreases to 10% towards the top of this interval. The lower part of the plateau phase of d 13 Cis characterized by the presence of impoverished calcareous nannofloras, with extremely low diversity (between seven and nine taxa) and abundance (between 0.4 and 0.7), and high percentages of Watznaueria barnesiae (70 75%). Eprolithus floralis decreases, while Biscutum constans, Zeugrhabdotus erectus and Cyclagelosphaera margerelii sharply decreased in the lower part of the plateau and temporarily vanished. A decreasing trend is also observed in the distribution of Prediscosphaera spp. and E. turriseiffelii, while Thoracosphaera decreases from about 13 to 3%. In contrast, the upper part of the plateau phase includes a recovery of nannofloras, as also shown by the increased diversity (up to 27 taxa) and abundance (averaging 2.4 specimens/fov), and the slight decline in abundance of Watznaueria barnesiae down to 60%. The relative abundance of E. floralis slowly increases from 10 to almost 15%. All of the highfertility proxies recur jointly in the nannofossil assemblages but with low frequency; that is, B. constans at around 0.6%, Z. erectus up to 2.4% and C. margerelii up to 4.7%. Both Prediscosphaera spp. and Eiffellithus turriseiffelii show an increasing trend. Thoracosphaera spp. shows a significant decline, from 12 to less than 2%, towards the top of the plateau phase of d 13 C. The onset of the biotic recovery starts in Arobes below the FO of the nannofossil Quadrum gartneri, when d 13 C starts to decline gently (sample ARO23-25). The recovery and post-excursion phases of d 13 C comprise a rebound of calcareous nannofloras, more pronounced towards the top of the studied section. The post-excursion phase shows diversity and abundance values similar to those recorded in the pre-excursion phase of d 13 C. Similar increases occur in percentages of nannofloral taxonomic groups, except for two taxa: B. constans, which remains in small amounts below 2%; and Thoracosphaera spp, which continuously decreases and almost vanished towards the top of the studied section, with an abundance of below 0.5% (Fig. 6). Nannofloral preservation and reworking. The preservation of the calcareous nannofossils varies from good to poor throughout the Arobes section. Only a few samples, located at the base of the studied succession within the UC3 zone (Figs 2 & 3), yielded very well-preserved specimens in an interval where Watznaueria barnesiae is below 40% (averaging 36%). Within this interval, the nannofloral taxa show little dissolution and/or overgrowth, therefore they are characteristically well preserved and the specimens could be identified to species level (up to 95%). Above sample ARO8 up to the middle part of the grey-greenish mudstone overlaying the black mudstone (ARO16/30-40), the preservation is moderate; some coccoliths are etched, dissolution and/or overgrowth are also present. Hence, specific identification is hindered in up to 25%. The beginning of this interval with moderate preservation is coincident, in the Arobes section, with a small increase in abundance of Watznaueria barnesiae, which represents more than 40% of total nannofloras. This interval of moderate nannofloral